Programming Robots with ROS by Morgan Quigley Brian Gerkey and William D. Smart

Author:Morgan Quigley, Brian Gerkey, and William D. Smart
Language: eng
Format: mobi, epub
Tags: COMPUTERS / Computer Science
ISBN: 9781491914595
Publisher: O’Reilly Media
Published: 2015-05-31T16:00:00+00:00

Actuation: Manipulator Arm

Many robots need to manipulate objects in their environment. For example, packing or palletizing robots sit on the end of production lines, grab items coming down the line, and place them into boxes or stacks. There is an entire domain of robot manipulation tasks called pick and place, in which manipulator arms grasp items and place them somewhere else. Security robot tasks include handling suspicious items, for which a strong manipulator arm is often required. An emerging class of personal robots hope to be genuinely useful in home and office applications, performing manipulation tasks including cleaning, delivering items, preparing meals, and so on.

Just as for mobile bases, the manipulator-arm subsystem has astonishing variety across robots, in response to the many tradeoffs made to support particular application domains and price points.

Although there are exceptions, the majority of manipulator arms are formed by a chain of rigid links connected by joints. The simplest kind of joints are single-axis revolute joints (also called “pin” joints), where one link has a shaft which serves as the axis around which the next link rotates, in the same way that a typical residential door rotates around its hinge pins. However, linear joints (also called prismatic joints) are also common, in which one link has a slide or tube along which the next link travels, just as a sliding door runs sideways back and forth along its track.

A fundamental characteristic of a robot manipulator is the number of degrees of freedom (DOF) of its design. Often, the number of joints is equal to the number of actuators; when those numbers differ, typically the DOF is taken to be the lower of the two numbers. Regardless, the number of DOF is one of the most significant drivers of manipulator size, mass, dexterity, cost, and reliability. Adding DOF to the distal (far) end of a robot arm typically increases its mass, which requires larger actuators on the proximal (near) joints, which further increases the mass of the manipulator.

In general, six DOF are required to position the wrist of the manipulator arm in any location and orientation within the workspace, providing that each joint has full range of motion. However, achieving full (360-degree) range of motion on all joints of a robot is often difficult to achieve at reasonable cost, due to constraints in mechanical structures, electrical wiring, and so on. As a result, seven-DOF arms are often used. The seventh DOF provides an “extra” degree of freedom which can be used to move the links of the arm while maintaining the position and orientation of the wrist, much as a human arm can move its elbow through an arc segment while maintaining the wrist in the same position.

Robots intended for manipulation tasks in human environments often have human-scale, 7-DOF arms, quite simply because the desired workspaces are human-scale surfaces, such as tables, countertops, and so on. In contrast, robots intended for industrial applications have wildly varying dimensions depending on the task they are to perform.

So far, we

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